see about:buildconfig  (also seen from about:support)
in current firefox build (not the hg one)

about:buildconfig
Build Machine

foutrelis
Build platform
target
x86_64-unknown-linux-gnu
Build tools
Compiler  Version   Compiler flags
gcc   4.9.2   -Wall -Wpointer-arith -Wdeclaration-after-statement -Werror=return-type -Werror=int-to-pointer-cast -Wtype-limits -Wempty-body -Wsign-compare -Wno-unused -Wcast-align -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong --param=ssp-buffer-size=4 -std=gnu99 -fgnu89-inline -fno-strict-aliasing -fno-math-errno -pthread -pipe
c++   4.9.2   -D_FORTIFY_SOURCE=2 -Wall -Wpointer-arith -Woverloaded-virtual -Werror=return-type -Werror=int-to-pointer-cast -Werror=type-limits -Wempty-body -Wsign-compare -Wno-invalid-offsetof -Wcast-align -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong --param=ssp-buffer-size=4 -fno-exceptions -fno-strict-aliasing -fno-rtti -fno-exceptions -fno-math-errno -std=gnu++0x -pthread -pipe -DNDEBUG -DTRIMMED -fprofile-use -fprofile-correction -Wcoverage-mismatch -O3 -fomit-frame-pointer -D_FORTIFY_SOURCE=2
Configure arguments

--enable-application=browser --prefix=/usr --libdir=/usr/lib --enable-official-branding --with-system-nspr --with-system-nss --with-system-jpeg --with-system-zlib --with-system-bz2 --with-system-png --with-system-libevent --with-system-libvpx --with-system-icu --enable-system-hunspell --enable-system-sqlite --enable-system-ffi --enable-system-pixman --enable-startup-notification --enable-pulseaudio --enable-gstreamer=1.0 --disable-crashreporter --disable-updater --disable-installer --disable-debug-symbols --with-google-api-keyfile=/build/firefox/src/mozilla-release/google-api-key

-----------
had to clean working folder due to a missing --incremental on ./inst which deleted some of it
now stuck on this error:
configure: error: Your toolchain does not support C++0x/C++11 mode properly. Please upgrade your toolchain


TODO: https://www.dnssec-tools.org/wiki/index.php/Firefox
add that patch

